Using Mozilla code in other projects

There are several ways you can use Mozilla code in your own project.

Mozilla provides a development platform for desktop applications, called XUL, atop which you can build applications.
Various components of the platform, such as the SpiderMonkey JavaScript engine, can be used in your own projects without the rest of the platform.
There are also modules that aren't used in Firefox but are available for use by other applications; an example of this is Rhino, the JavaScript engine written in Java.
You can embed Gecko, the layout engine used by Firefox to process web pages, as a browser within your own application.

Building XUL applications

The Joy of XUL: An introduction to XUL; a must read for new developers.
XUL Reference: The complete reference to XUL.
XULRunner: A Mozilla runtime package that can be used to bootstrap XUL and XPCOM applications with ease.
Mozilla Toolkit: Information about the Mozilla Toolkit API.
Signing Mozilla apps for Mac OS X: How to code-sign applications based on Mozilla code for Mac OS X's application security model.

SpiderMonkey: SpiderMonkey is the JavaScript runtime engine used by Mozilla projects.
NSPR: The Netscape Portable Runtime provides a platform-neutral API for system level and libc-type functions.
Necko: The Mozilla networking library provides cross-platform networking functionality.

For information on embedding a web browser into your own application see Embedding Mozilla.